The aims of this protocol are: (1) to compare oral desensitization to trimethoprim-sulfamethoxazole over 8 days with no desensitization (full dose of trimethoprim-sulfamethoxazole in patients with AIDS who have had a history of adverse reaction to trimethoprim-sulfamethoxazole) and (2) to assess the acetylation status in these patients as a predictor of adverse reactions to the re-challenge with trimethoprim-sulfamethoxazole.